With the advent of digitization of images, digital image distribution and digital video availability, copyright protection of such digital imagery has become a substantial issue for image publishers and authors. One technique used to identify digital image ownership is a digital "watermark" that is embedded into an image or image sequence. Such watermarks must be secure, robust to intentional corruption and to compression processing, not unreasonably complex to embed and extract, and compatible and interoperable with conventional image processing systems. The watermark is generally invisible to a viewer. However, in some applications, it is desirable to produce a visible watermark that can be removed by an authorized image decoder and that can not be removed by an unauthorized decoder.
Various digital watermarking techniques have been attempted for both still and video images with varying levels of success. The use of spread-spectrum techniques are disclosed in Hartung et al., "Digital Watermarking of Raw and Compressed Video", Systems for Video Communication, October 1996, pp. 205-213 and Hartung et al., "Watermarking of MPEG-2 Encoded Video Without Decoding and Re-encoding", Proceedings of SPIE 3020, Multimedia Computing and Networking 97 (MMCN 97), February 1997. The first of these papers discloses a technique that spreads the energy of a watermark image throughout a video sequence to be "watermarked" using a pseudo-noise signal. Once the pseudo-noise signal has been embedded into the video sequence, the system encodes the video sequence containing the watermark. In this manner, the watermarking is accomplished in the pixel domain. As such, any coding losses in the video coding process apply to the watermark as well as the images in the video sequence. At the decoder, the spread watermark is correlated and extracted from the video sequence. The use of a spread-spectrum technique requires substantial synchronization and signal processing hardware at both the encoder and decoder to facilitate recovery of the watermark and the video. As such, pixel domain watermark processes are generally avoided.
Both of the aforementioned papers disclose a bitstream domain watermarking technique where the "block" of an image frame within the video sequence is coded and then combined with a coded watermark signal. Specifically, a block is generally an 8.times.8 pixel portion of an image frame. The block is coded using a discrete cosine transform (DCT) to form a coded block. A watermark image is similarly divided into blocks and DCT coded. The DCT coefficients representing the coded watermark block and the coded image block are then added together to form a combined block. The combined block is quantized and error coded. Thereafter, a selection process is performed to transmit only the "watermarked" coefficients that will not increase the bit rate necessary to transmit the encoded video sequence. This selection process compares the number of bits required to encode coefficients of the combined block with the number of bits required to encode coefficients of the image block only. If the number of combined block bits is greater than the number of image block bits, the system transmits the image block bits; otherwise, the combined block is transmitted. Clearly, such a selection process eliminates some of the watermark information and thus distorts the watermark. In some extreme cases, the watermark information may not be transmitted at all or such a small amount may be transmitted that the watermark is rendered useless.
Therefore, a need exists in the art for a watermarking technique that does not remove any amount of watermark signal to facilitate watermarking a video sequence without exceeding a predefined bit budget.